JPH10320755A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase reproducing output and to decrease bias noise by specifying the compounding ratio of a first magnetic powder having a large specific surface area and a second magnetic powder having a rather small specific surface area and mixing the powders to form a single magnetic layer. SOLUTION: Because the first magnetic powder has a rather large specific surface area (small size), the amt. of the powder per unit volume can be decreased, which is suitable to realize high-density recording and to decrease bias noise. Because the second magnetic powder has a rather small specific surface area (large size), it has excellent orienting property and dispersibility and is suitable to increase reproducing output. The first magnetic powder having 40 to 45 m<2> /g specific surface area is mixed with the second magnetic powder having 30 to 35 m<2> /g specific surface area by 20:80 to 40:60 weight ratio, and the mixture is applied on a nonmagnetic supporting body 2 to form a single magnetic layer 3. By optimizing the compounding balance of the first magnetic powder and the second magnetic powder, performances above described can be obtd. The difference of the coercive force between the first and second magnetic powders is preferably <=3 kA/m.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium (for example, a magnetic tape or a magnetic disk) having a magnetic layer composed of a magnetic powder and a binder.

[0002]

2. Description of the Related Art Conventionally, in the field of magnetic recording, oxide magnetic powders such as ferrite and chromium oxide, Fe, Co, N
A magnetic paint obtained by dispersing a metal magnetic powder or an alloy magnetic powder mainly composed of i or the like in a binder made of an organic polymer material is used as a polyester or polyethylene terephthalate (PET)
A coating-type magnetic recording medium having a magnetic layer formed by coating on a non-magnetic support made of, for example, is widely used in terms of versatility and productivity.

In recent years, demands for higher density recording (short wavelength recording) and higher quality characteristics (sound quality, image quality, etc.) have been more and more increased with respect to such a coating type magnetic recording medium. In order to satisfy these requirements, the electromagnetic conversion characteristics must be improved, especially, the maximum output level (MOL: Ma
It is necessary to improve the ximum output level (that is, to improve the reproduction output) and to reduce bias noise (bias noise).

[0004]

In order to reduce the bias noise and to improve the maximum output level, the size of the magnetic powder is reduced to increase the number of magnetic powders in a unit volume, and then the dispersibility of the magnetic powder is increased. And it is necessary to improve the orientation.

However, simply reducing the size of the magnetic powder (ie, increasing the specific surface area of the magnetic powder)
Then, the bias noise can be reduced, but the orientation and the dispersibility of the magnetic powder in the process of forming the magnetic layer may be deteriorated, and the output level may be reduced. In addition, there is a problem in practical use, for example, the transfer characteristic deteriorates due to an increase in the low coercive force component.

On the other hand, in order to improve the orientation and dispersibility of the magnetic powder in the process of forming the magnetic layer and increase the maximum output level, the size of the magnetic powder is increased (that is, the specific surface area of the magnetic powder is reduced). ), The orientation and dispersibility are improved, and the maximum output level can be increased, but the bias noise increases, and it is difficult to obtain a magnetic recording medium suitable for high-density recording.

As described above, it is difficult to achieve both output characteristics and bias characteristics uniquely in the magnetic layer depending on the size of the magnetic powder. This is particularly remarkable in a single magnetic layer.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional circumstances, and has as its object to provide a magnetic recording medium having excellent magnetic characteristics such as a large reproduction output and a small bias noise and excellent electromagnetic conversion characteristics. Is to do.

[0009]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, in a magnetic recording medium having a magnetic layer comprising a magnetic powder and a binder, By including a plurality of types of magnetic powders having different specific surface area ranges and limiting the compounding ratio of these magnetic powders, the reproduction output (that is, the maximum output level:
It has been found that a magnetic recording medium having excellent magnetic characteristics and electromagnetic conversion characteristics can be obtained, for example, the same is large and bias noise is small.

That is, the present invention relates to a magnetic recording medium having a magnetic layer composed of a magnetic powder and a binder, wherein the first magnetic powder having a specific surface area of 40 to 45 m ² / g and the specific surface area of 30
Weight ratio of 20:80 with the second magnetic powder of up to 35 m ² / g.
The present invention relates to a magnetic recording medium (hereinafter, referred to as a magnetic recording medium of the present invention) characterized by being mixed within the range of ~ 40: 60 and contained in the magnetic layer.

According to the magnetic recording medium of the present invention, the content per unit volume can be increased since the specific surface area is relatively large (that is, its size is small), and high density recording and reduction of bias noise can be achieved. Mixing balance between the suitable first magnetic powder and the second magnetic powder suitable for improving reproduction output because of its relatively small specific surface area (that is, its large size), so that it has excellent orientation and dispersibility. Is optimized, it is possible to obtain a magnetic recording medium having excellent magnetic characteristics and electromagnetic conversion characteristics such as high reproduction output and low bias noise.

Here, the above specific surface area is a specific surface area based on a measuring method by the BET method, and is measured using an "specific surface area automatic measuring device" manufactured by Shimadzu Corporation.

Next, the reasons for limiting the above mixing ratio and specific surface area will be described.

If the content ratio of the first magnetic powder having a relatively large specific surface area is less than 20 by weight (that is, if the content ratio of the second magnetic powder having a relatively small specific surface area is 80, The content ratio of the second magnetic powder relatively increases, and the orientation and dispersibility can be sufficiently ensured and the reproduction output can be improved, but the specific surface area of the magnetic powder as a whole is increased. Since it becomes smaller (that is, its size becomes larger), the bias noise becomes larger.

When the content ratio of the first magnetic powder exceeds 40 by weight (that is, when the content ratio of the second magnetic powder is less than 60), bias noise can be reduced. However, sufficient orientation and dispersibility cannot be secured, and sufficient reproduction output cannot be obtained.

Further, regarding the specific surface area of the first magnetic powder, if the specific surface area of the magnetic powder exceeds 45 m ² / g, the magnetic powder is liable to agglomerate, the dispersibility is remarkably deteriorated, and the reproduction output, etc. Decrease. In addition, this specific surface area is 4
If it is less than 0 m ² / g, the effect of reducing bias noise will be small.

Further, regarding the specific surface area of the second magnetic powder, when the specific surface area of the magnetic powder exceeds 35 m ² / g, the dispersibility of the whole magnetic paint is deteriorated, and the reproduction output and the like are reduced. If the specific surface area is less than 30 m ² / g, the total amount of magnetic powder per unit volume is reduced, and the effect of reducing bias noise is reduced.

As described above, the specific surface area of the magnetic powder and the magnetic characteristics and electromagnetic conversion characteristics that vary depending on the compounding ratio are determined by comparing the specific surface area with the first magnetic powder having a specific surface area of 40 to 45 m ² / g by 30 to 45 m ² / g. By limiting the weight ratio of the second magnetic powder of 35 m ² / g to the range of 20:80 to 40:60 by weight, it is possible to improve the reproduction output and reduce the bias noise for the first time, and obtain the magnetic characteristics and the electromagnetic characteristics. A magnetic recording medium with excellent conversion characteristics can be obtained. Here, the weight ratio at the time of the compounding is more preferably from 25:75 to 35:65.

In the magnetic recording medium of the present invention, the difference in coercive force between the first magnetic powder and the second magnetic powder is 3
It is desirably at most kA / m.

When the difference in the coercive force increases, the ratio of the magnetic powder having a small coercive force increases to cause magnetic transfer while the medium having the recording signal is wound and held, or the magnetic powder having a large coercive force can The erasure of the recorded magnetization may become difficult due to an increase in the ratio. For this reason, both coercive forces (Hc)
(Ie, ΔHc) is less than 3 kA / m (about 37.7
Oe).

In the magnetic recording medium of the present invention,
It is fully possible for the magnetic layer to be a single-layer magnetic layer.

In general, for the purpose of improving reproduction output and reducing bias noise, a multi-layer structure in which a plurality of magnetic layers are provided on a non-magnetic support, and each magnetic layer contains a magnetic powder of a different type and size. Although a magnetic recording medium is known, a plurality of magnetic layers must be provided on a nonmagnetic support, which complicates the manufacturing process. Further, in a magnetic recording medium having a single magnetic layer, a magnetic layer containing one type of magnetic powder is generally provided on a non-magnetic support.

On the other hand, in the magnetic recording medium of the present invention, even if the magnetic layer having the characteristic configuration of the present invention is a single layer, the improvement in the reproduction output described above is achieved based on the characteristic configuration of the present invention. , And the reduction of bias noise can be sufficiently achieved. In addition, a single layer is particularly effective for obtaining the effects of the present invention.

Here, in the magnetic recording medium of the present invention, the first magnetic powder and the second magnetic powder having different specific surface areas may be of the same type or different types. It does not matter. If the specific surface area of the magnetic powder to be used is within the above range, a different kind of magnetic powder may be used as the first magnetic powder (or the second magnetic powder).

The specific surface areas of the first magnetic powder and the second magnetic powder may be slightly different before and after kneading and mixing. Even if these magnetic powders are crushed during the above treatment and the specific surface area of the magnetic powder is out of the above range, most of the magnetic powders are substantially in the above specific surface area range. If present, a small amount of magnetic powder having another specific surface area may be included.

The magnetic layer is a single layer having a thickness of, for example, 3.5 to 3.5 in the case of an audio magnetic tape.
Preferably, the thickness is 5.0 μm.

The magnetic recording medium of the present invention is a so-called coating type magnetic recording medium in which a magnetic coating film comprising the magnetic powder and a binder is formed on a non-magnetic support.

The mixing ratio of the magnetic powder to the binder is preferably 12 to 33 parts by weight, more preferably 16 to 20 parts by weight, based on 100 parts by weight of the binder. More preferred.

In the magnetic recording medium of the present invention, any of the materials usually used in this type of magnetic recording medium can be used. Hereinafter, materials usable for the magnetic recording medium of the present invention will be exemplified, but of course, the materials are not limited to these materials.

Examples of the nonmagnetic support include a polymer support formed of a polymer material represented by polyethylenes, vinyl resins, polyesters, polyolefins, celluloses, polyimides and polycarbonates; Alloy, metal support made of titanium alloy, etc., ceramic support made of aluminum glass, etc.,
Glass supports and the like can be mentioned.

The magnetic powder is preferably a ferromagnetic powder, and examples thereof include an iron oxide-based ferromagnetic powder, a chromium oxide-based ferromagnetic powder, a metal-based (including alloy) ferromagnetic powder, and a hexagonal ferrite powder. . Needless to say, these magnetic powders have the specific surface area and the compounding amount as described above.

Examples of the binder include polymers such as vinyl chloride, vinyl acetate, vinyl alcohol, vinylidene chloride, acrylates, methacrylates, styrene, butadiene and acrylonitrile, or a combination of two or more of these polymers. And a polyurethane resin, a polyester resin, an epoxy resin and the like.

The magnetic layer containing the magnetic powder and the binder as main components is preferably prepared by mixing and dispersing the magnetic powder and the binder in an organic solvent with the non-magnetic support. It is formed by coating on top. The mixing and dispersing steps and the coating step can use ordinary steps.

As the organic solvent for mixing and dispersing the magnetic powder and the binder, commonly used organic solvents can be used, for example, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; Esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate and glycol monoethyl ether; glycol ethers such as glycol dimethyl ether, glycol monoethyl ether and dioxane; aromatic hydrocarbons such as benzene, toluene and xylene; Hexane, aliphatic hydrocarbons such as heptane, methylene chloride, ethylene chloride, carbon tetrachloride,
Chlorinated hydrocarbons such as chloroform, ethylene chlorohydrin and dichlorobenzene are exemplified.

The basic structure of the magnetic recording medium of the present invention has been described above. Like the usual coating type magnetic recording medium, in addition to the binder and the magnetic powder, for example, an abrasive, a dispersant Known additives such as a lubricant, an antistatic agent and a rust preventive may be added.

The magnetic recording medium may be provided with a back coat layer, a top coat layer, etc., if necessary. The film forming conditions and composition of the back coat layer, the top coat layer and the like may be the conditions and compositions usually used in this type of magnetic recording medium, and are not particularly limited.

Next, an example of a magnetic recording medium according to the present invention will be described with reference to FIG.

FIG. 1 shows the magnetic powders 4a and 4b and the binder 5
FIG. 3 is a schematic sectional view of a main part of a magnetic recording medium 1 in which a magnetic layer 3 is provided on a non-magnetic support 2.

The magnetic recording medium 1 is, for example, a magnetic tape. On the non-magnetic support 2, a magnetic powder 4a as the first magnetic powder and a magnetic powder 4b as the second magnetic powder are provided. Magnetic layer 3 mixed and dispersed in binder 5
have.

Since the magnetic powders 4a and 4b have the above specific surface area in the above mixing ratio, they are uniformly dispersed in the binder 5 and have excellent orientation. That is, since the magnetic powder 4a has a relatively large specific surface area, the content of the magnetic powder per unit volume can be increased, and it is suitable for high-density recording and reduction of bias noise. Since the magnetic powder 4b having a small surface area has an appropriate amount, it has excellent orientation and dispersibility, and is suitable for improving the reproduction output. Therefore, the magnetic recording medium 1 has a large reproduction output and a small bias noise.

Generally, in a magnetic recording medium in which a magnetic layer mainly composed of a magnetic powder and a binder is provided on a non-magnetic support, the size of the magnetic powder must be reduced to reduce bias noise. Therefore, it is necessary to increase the number of magnetic powders per unit volume. However, simply increasing the specific surface area of the magnetic powder can lower the bias noise.However, the reproduction output is small because the orientation and dispersibility of the magnetic powder during the production of the magnetic recording medium are deteriorated. There was a problem of becoming.

Therefore, according to the magnetic recording medium of the present invention described above, the magnetic powder to be used is a magnetic powder having good orientation and dispersibility and a small specific surface area (second magnetic powder).
By mixing magnetic powder (first magnetic powder) having a specific surface area useful for reducing bias noise and improving reproduction output, the magnetic properties in a unit volume can be maintained while maintaining high orientation (and dispersibility). The number of powders can be increased. This makes it possible to obtain a magnetic recording medium having excellent magnetic characteristics and electromagnetic conversion characteristics and low bias noise.

[0043]

EXAMPLES The present invention will be described below with reference to specific examples, but the present invention is not limited to the following examples.

First, a magnetic paint having the following composition was prepared.

<Magnetic coating composition> Magnetic powder (Co-γ-Fe ₂ O ₃ ) 100 parts by weight * However, the mixing ratio, specific surface area, etc. are described in Table 1 below. 8.5 parts by weight of vinyl chloride copolymer Parts Polyester polyurethane 8.5 parts by weight Silane coupling agent 1 part by weight Alumina powder 4 parts by weight Carbon 2 parts by weight Myristic acid 0.2 parts by weight Stearic acid ester 0.4 parts by weight Methyl ethyl ketone 100 parts by weight Toluene 100 parts by weight Cyclohexanone 50 parts by weight Department

According to the above composition, each magnetic coating composition was weighed, mixed and dispersed by a sand mill to prepare a magnetic coating. The magnetic powder to be used was separately mixed with a binder in advance and mixed with other components to form a coating.

The magnetic paint thus prepared was
After adding a part by weight of an isocyanate-based curing agent (Coronate L manufactured by Nippon Polyurethane Co.), the composition is applied to a non-magnetic support (polyethylene terephthalate) having a thickness of 12.0 μm and dried to form a magnetic layer. The surface of the magnetic layer was subjected to a calendering treatment to obtain a raw medium (magnetic film). This was cut to a 1/8 inch width to produce audio magnetic tapes (sample tapes) of Examples 1 to 21 having a magnetic layer containing magnetic powder having the physical properties shown in Table 1 below. However, the thickness of this magnetic layer is 5.0 μm
And Further, ΔHc in Table 1 indicates a difference in coercive force Hc between the first magnetic powder and the second magnetic powder.

[0048]

[0049]

Next, about the obtained sample tape,
The squareness ratio, reproduction output, bias noise and erasure were evaluated.

Squareness ratio: Squareness ratio (Br / Bm: Br = residual magnetization, B) using a sample vibration magnetometer (manufactured by Toei Kogyo KK).
m = saturated magnetization) was measured.

Reproduction output (maximum output level): IEC94
The reproduction output was measured based on -5 (international standard for electromagnetic conversion characteristics of audio cassette) and JIS C 5566 (national standard for audio cassette). The reproduced output is MOL315 (31 at the reference bias).
The output level at which the third harmonic becomes 3% after recording 5 Hz was substituted.

Bias noise: Cassette deck TC-K
Using A5ES (manufactured by Sony Corporation), the signal generator
Using HP3314A (manufactured by Hewlett-Packard), the signal output section was input to a bandpass filter (Type 1617 manufactured by B & K) at a measurement frequency of 20 to 20 KHz to measure bias noise. At this time, 1/3
An octave filter was used in the band to be measured. In addition, the total value is based on JI according to JIS C5566.
The S auditory correction A curve was used. Note that the dB value is 0 dB
= 250 nWb / m. The values in Table 2 below are values for a reference tape (manufactured by Sony Corporation).

Erasing (Erasing ability of recording magnetization): Using a deck having a recording head gap of 4 μm and a reproducing head gap of 1 μm, using a signal generator HP3311A (manufactured by Hewlett-Packard Company) at a specified bias, 3
Enter a 15 Hz MOL value. Thereafter, a current was applied to the erasing head using a 1/3 octave filter, and a deviation from the reference tape at the time of erasing at 65 dB was obtained.

These measurement results are shown in Table 2 below and FIGS.
As shown in FIG. FIG. 2 is a graph showing the change in the maximum output level and the bias noise depending on the mixing ratio of the first magnetic powder (the ratio in 100 parts by weight of the magnetic powder).
Is a graph showing the change of the maximum output level and the bias noise according to the specific surface area of the first magnetic powder, and FIG. 4 is a graph showing the change of the maximum output level and the bias noise according to the specific surface area of the second magnetic powder. It is a graph. Example 1
4. With respect to the sample tapes of Examples 17 and 21, the erasing ability of recorded magnetization was measured, and the measurement results are shown in Table 3 below.

[0056]

[0057]

Next, each of the above sample tapes is evaluated with reference to Tables 1 and 2 and FIGS.

FIG. 2 is a plot of the evaluation results of the sample tapes of Examples 1 to 11, and
When the mixing ratio between the first magnetic powder and the second magnetic powder was changed using 5 m ² / g of the first magnetic powder and the ^second magnetic powder having a specific surface area of 32 m ² / g, 5 is a graph showing changes in the maximum output level and bias noise.

From this graph, it can be seen that the sample in which the mixing ratio of the first magnetic powder is 20 to 40, that is, the mixing ratio (weight ratio) of the first magnetic powder and the second magnetic powder is 20:80 to 40:60. It can be seen that the tapes (Examples 4 to 8) are excellent in improving the maximum output level and reducing the bias noise. Table 2 also shows that a sufficient value was obtained for the squareness ratio.

When the mixing ratio of the first magnetic powder is less than 20, the maximum output level, which is considered to be due to the good dispersibility and orientation of the magnetic powder and the coercive force, is greatly improved. However, bias noise, which is considered to be due to the small specific surface area of the magnetic powder (that is, the size of the magnetic powder is large), is increasing. On the other hand, when the mixing ratio of the first magnetic powder exceeds 40, the bias noise is greatly reduced with the miniaturization of the magnetic powder, but the maximum output level is greatly deteriorated.

From the graph and Table 2, the mixing ratio of the first magnetic powder is 25 to 35 [that is, the mixing ratio of the first magnetic powder and the second magnetic powder = 25: 75 to 35:65].
Is more preferable.

FIG. 3 is a plot of the evaluation results of the sample tapes of Example 6 and Examples 12 to 15.
6 is a graph showing changes in the maximum output level and bias noise depending on the specific surface area of the first magnetic powder. However, the specific surface area of the second magnetic powder is 32 m ² / g, and the mixing ratio between the first magnetic powder and the second magnetic powder is 30:70 by weight.

From the graph, it is found that the specific surface area of the first magnetic powder is 40 to 45 m ² / g (Example 6, Example 1).
3, it can be seen that Example 14) is superior in improving the maximum output level and reducing the bias noise. Table 2
This shows that an excellent value was also obtained for the squareness ratio.

In particular, a comparison was made between Example 6 (a sample tape having a specific surface area of the first magnetic powder of 45 m ² / g) and Example 15 (a sample tape having a specific surface area of the first magnetic powder of 49 m ² / g). As can be seen from both figures, the bias noise is reduced in both cases, but the sample tape of Example 15 shows a decrease in dispersibility and orientation due to a large specific surface area, and a decrease in the maximum output level due to a decrease in coercive force. Deterioration has occurred.

FIG. 4 is a plot of the evaluation results of the sample tapes of Example 6 and Examples 16 to 19.
9 is a graph showing changes in the maximum output level and bias noise depending on the specific surface area of the second magnetic powder. However, the specific surface area of the first magnetic powder is 45 m ² / g, and the mixing ratio between the first magnetic powder and the second magnetic powder is 30:70 by weight.

From this graph, it is found that the specific surface area of the second magnetic powder is 30 to 35 m ² / g (Example 6, Example 1).
7, Example 18) is excellent in improving the maximum output level and reducing the bias noise. Table 2
From this, it can be seen that excellent values were also obtained for the squareness ratio.

Next, from Table 3, the difference (ΔHc) in the coercive force between the first magnetic powder and the second magnetic powder is 3.0 kA / m
It is understood that the following is preferable. The sample tapes of Examples 14 and 17 in which the difference in coercive force (ΔHc) is in such a range are particularly excellent in the erasing ability, but the sample tape having ΔHc of 4.4 (Example 21) has the erasure of the recorded magnetization. Is not enough.

When the sample tape of Example 2 and the sample tape of Example 20 are compared, the maximum output level is excellent because both have a large amount of the second magnetic powder, but the difference in specific surface area (Example) 2 sample tape is 32m ² /
g, 26 m ² / g of the sample tape of Example 20) shows that the ability to reduce the bias noise is different.

[0070]

The magnetic recording medium of the present invention is a magnetic recording medium having a magnetic layer comprising a magnetic powder and a binder, wherein the first magnetic powder having a specific surface area of 40 to 45 m ² / g has a specific surface area of 40 to 45 m ² / g. 30 to 35 m ² / g of a second magnetic powder is mixed in a weight ratio of 20:80 to 40:60 and contained in the magnetic layer, and has a relatively large specific surface area. Therefore, the content per unit volume can be increased, and the first magnetic powder suitable for reduction of bias noise and high-density recording, and relatively small specific surface area, so that the orientation and dispersibility are excellent, and the reproduction output Since the composition balance with the second magnetic powder suitable for improvement of the magnetic recording medium is optimized, it is possible to obtain a magnetic recording medium having excellent magnetic characteristics such as high reproduction output and low bias noise and excellent electromagnetic conversion characteristics. .

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of an example of a magnetic recording medium of the present invention.

FIG. 2 is a graph showing a change in a maximum output level and a bias noise depending on a mixing ratio of a first magnetic powder according to the present invention.

FIG. 3 is a graph showing changes in the maximum output level and bias noise depending on the specific surface area of the first magnetic powder.

FIG. 4 is a graph showing changes in the maximum output level and the bias noise depending on the specific surface area of the second magnetic powder.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Magnetic recording medium, 2 ... Non-magnetic support, 3 ... Magnetic layer, 4
a, 4b: magnetic powder, 5: binder

Claims (3)

[Claims] 1. A magnetic recording medium having a magnetic layer comprising a magnetic powder and a binder, having a specific surface area of 40 to 45 m ^2.
/ G of the first magnetic powder and a specific surface area of 30 to 35 m ² / g
And a second magnetic powder in a weight ratio of 20:80 to 40:60, and contained in the magnetic layer. 2. The magnetic recording medium according to claim 1, wherein a difference in coercive force between the first magnetic powder and the second magnetic powder is 3 kA / m or less. 3. The magnetic recording medium according to claim 1, wherein said magnetic layer is a single layer.